Shield film, shielded printed wiring board, and method for manufacturing shield film

ABSTRACT

To provide a shield film which is capable of suitably shielding electric field waves, magnetic field waves, and electromagnetic waves progressing from one side to the other side of the shield film and has good transmission characteristics, a shielded printed wiring board, and a method for manufacturing the shield film, a metal layer  3  which is 0.5 μm to 12 μm thick and an anisotropic conductive adhesive layer  4  which is anisotropic so as to be electrically conductive only in thickness directions are provided in a deposited manner, so that electric field waves, magnetic field waves, and electromagnetic waves progressing from one side to the other side of the shield film are suitably shielded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Stage of PCT application numberPCT/JP2012/076473 filed Oct. 12, 2012. The PCT application was based onand claims priority to underlying Japanese priority application numberJP 2011-256816 filed Nov. 24, 2011, the contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a shield film used in an apparatus suchas a mobile apparatus, a shielded printed wiring board using the shieldfilm, and a method for manufacturing the shield film.

BACKGROUND ART

A shield film for shielding noise such as electromagnetic noise has beenpublicly known. For example, PTL 1 and 2 recite a shield film for aprinted wiring board, in which a metal layer and an adhesive layer areserially deposited.

CITATION LIST Patent Literature

PTL 1

Japanese Unexamined Patent Publication No. 2005-276873

PTL 2

Japanese Unexamined Patent Publication No. 2009-246121

SUMMARY OF INVENTION Technical Problem

Mobile apparatuses represented by smart phones have becomemulti-functional. For example, large-capacity signal processing isrequired to achieve not only the Internet connection but also thereproduction of high-definition images, high-quality images, andthree-dimensional images and the processing acceleration. To processsuch large-capacity signals, the signal processing is accelerated andthe noise reduction in signal lines and good transmissioncharacteristics of signals are required. For this reason, a flexibleprinted wiring board having better shielding characteristics andtransmission characteristics has been demanded.

To solve the problem above, an object of the present invention is toprovide a shield film which is capable of suitably shielding electricfield waves, magnetic field waves, and electromagnetic waves progressingfrom one side to the other side of the shield film and has goodtransmission characteristics, a shielded printed wiring board, and amethod for manufacturing the shield film.

Solution to Problem

The present inventors diligently made extensive studies to achieve theobject above and found that good shielding characteristics andtransmission characteristics were achieved by arranging a metal layer tohave a thickness of 0.5 μm to 12 μm and using an anisotropic conductiveadhesive for an adhesive layer.

That is to say, in the present invention, a metal layer which is 0.5 μmto 12 μm thick and an anisotropic conductive adhesive layer, which arein a deposited state.

According to the arrangement above, because of the presence of the metallayer 0.5 μm to 12 μm thick, electric field waves, magnetic field waves,and electromagnetic waves progressing from one side to the other side ofthe shield film are suitably shielded. Furthermore, because theconductive adhesive layer is an anisotropic conductive adhesive layer,the transmission characteristics are better than those in case of aisotropic conductive adhesive layer.

In the shield film of the present invention, the metal layer is metalfoil.

According to the arrangement above, a metal layer with a desiredthickness is easily obtained and better shielding characteristics areachieved as compared to a case where a thin-film metal layer is formedby vapor deposition.

In the shield film of the present invention, the metal foil is formed byrolling.

According to this arrangement, on account of good shape retainingproperty, the workability when assembling a flexible substrate or thelike with which the shield film is bonded is improved.

In the shield film of the present invention, the thickness of the metalfoil is adjusted by etching.

According to this arrangement, after the metal foil is processed to havea thickness in the first size, the metal foil is etched to be as thin asthe second size. With this, it is possible to obtain a highly preciselythin metal layer which cannot be processed by rolling.

In the shield film of the present invention, the metal foil is mainlymade of copper.

According to the arrangement above, it is possible to obtain a shieldfilm having good workability on account of the good shape retainingproperty, at low cost.

The shield film of the present invention may be arranged such that, aprotective metal layer is provided between a metal layer which is themetal foil mainly made of copper and the anisotropic conductive adhesivelayer.

According to the arrangement above, the oxidization of the metal layeris restrained and the increase in the surface resistance of the metallayer is restrained, with the result that the shielding effect is stablyexerted.

The shield film of the present invention may be arranged such that themetal layer is formed by an additive process. This arrangement makes itpossible to finely adjust the thickness of the metal layer when formingthe metal layer.

The shield film of the present invention may be arranged such that, asthe additive process, the metal layer is formed by at least one ofelectroplating and electroless plating.

This arrangement makes it possible to finely adjust the thickness of themetal layer when forming the metal layer and to improve the productionefficiency.

In the shield film of the present invention, the shield film is used asan electromagnetic waves shield film of a signal transmission systemtransmitting a signal with a frequency of 10 MHz to 10 GHz.

This makes it possible to provide a shield film which is suitable forhigh-speed transmission but is low cost.

A shielded printed wiring board of the present invention includes: aprinted wiring board including a base member in which a printed circuitis formed and an insulating film provided on the base member so as tocover the printed circuit; and the above-described shield film providedon the printed wiring board.

In the shielded printed wiring board of the present invention, theprinted circuit includes a ground wiring pattern.

A method for manufacturing shield film of the present invention includesthe steps of: after forming metal foil with a predetermined thickness byrolling, etching the metal foil to have a predetermined thicknessfalling within the range of 0.5 μm to 12 μm; and forming an anisotropicconductive adhesive layer on one side of the metal layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross section of a shield film.

FIG. 2(a) is a cross section of a shielded printed wiring board whichincludes a signal circuit and a ground circuit in its wiring pattern.FIG. 2(b) is a cross section of a shielded printed wiring board whichincludes only a signal circuit in its wiring pattern.

FIG. 3 shows the structure of a system used in a KEC method. FIG. 3(a)shows an electric field wave shielding effect evaluator. FIG. 3(b) showsa magnetic field wave shielding effect evaluator.

FIG. 4 (a) shows results of measurement of the electric field waveshielding capability by the KEC method. FIG. 4(b) shows a result ofmeasurement of the magnetic field wave shielding capability by the KECmethod.

FIG. 5 is a system configuration diagram of a system for measuringfrequency characteristics.

FIG. 6(a) shows results of measurement of the frequency characteristicsin case of one-sided shielding. FIG. 6(b) shows results of measurementof the frequency characteristics in case of double-sided shielding.

FIG. 7 is a system configuration diagram of a system for measuringoutput wave characteristics.

FIG. 8(a) shows results of measurement of the output wavecharacteristics when the bit rate is 1.0 Gbps. FIG. 8(b) shows resultsof measurement of the output wave characteristics when the bit rate is3.0 Gbps.

FIG. 9 shows an experimental apparatus for measuring a shape retainingproperty.

FIG. 10 shows an experimental apparatus for measuring a slidingcharacteristic based on the IPC standard.

FIG. 11 is a cross section of a shielded printed wiring board includinga shield film having a protective metal layer.

DESCRIPTION OF EMBODIMENTS

The following will describe a preferred embodiment of the presentinvention with reference to figures.

(Structure of Shield Film 1)

A shield film 1 shown in FIG. 1 is structured in such a way that a metallayer 3 which is 0.5 μm to 12 μm thick and an anisotropic conductiveadhesive layer 4 are provided in this order on one surface of aninsulating layer 2. In other words, in the shield film 1, the metallayer 3 and the anisotropic conductive adhesive layer 4 are deposited.

(Insulating Layer 2)

The insulating layer 2 is constituted by layers such as a cover film anda coating layer made of insulating resin.

The cover film is made of engineering plastics. Examples of theengineering plastics include polypropylene, cross-linked polyethylene,polyester, polybenzimidazole, aramid, polyimide, polyimidoamide,polyetherimide, polyphenylene sulfide (PPS), and polyethylenenaphthalate (PEN).

A low-cost polyester film is preferred when the required degree of heatresistance is not high. A polyphenylene sulfide film is preferred whenflame retardance is required. An aramid film or a polyimide film ispreferred when good heat resistance is required.

The insulating resin is of any type as long as electric insulation isachieved. Examples of the insulating resin includes thermosetting resinand ultraviolet curing resin. Examples of the thermosetting resininclude phenol resin, acrylic resin, epoxy resin, melamine resin,silicon resin, and acryl-denatured silicon resin. Examples of theultraviolet curing resin include epoxy acrylate resin, polyesteracrylate resin, and methacrylate modification thereof. The curing is ofany type, e.g., thermosetting, ultraviolet curing, and electron beamcuring.

When the shield film 1 is employed in a flexible printed wiring board,the lower limit of the thickness of the insulating layer 2 is preferably1 μm, more preferably 3 μm. The upper limit of the thickness of theinsulating layer 2 is preferably 10 μm, more preferably 7 μm.

The metal layer 3 is 0.5 μm to 12 μm thick. This makes it possible tosuitably shield electric field waves, magnetic field waves, andelectromagnetic waves progressing from one side to the other side of theshield film, and such shielding property is preferable when the shieldfilm is employed in the flexible printed wiring board.

The metal layer 3 is preferably metal foil. With this, a metal layerwith a desired thickness is easily obtained, and the obtained shieldingcharacteristics are better than those of a thin-film metal layer formedby vapor deposition. In addition to the above, the metal layer 3 ispreferably formed by rolling. This provides the shield film with a goodshape retaining property. The workability when assembling a flexiblesubstrate or the like with which the shield film is bonded is improved.For example, when a flexible printed wiring board including the shieldfilm is bended and attached to a mobile apparatus or the like, theflexible printed wiring board retains the bended state on account of thegood shape retaining property of the shield film. The operator istherefore not required to retain the bended state by himself/herself,with the result that the assembling work of the mobile apparatus or thelike becomes less burdensome and the workability is improved. Inaddition to the above, when the metal layer 3 is formed by rolling, thelayer thickness is preferably adjusted by etching.

Preferably the metal layer 3 is mainly made of copperas a metalmaterial. With this, good workability and good conductivity are obtainedon account of the good shape retaining property, and the shield film ismanufactured at low cost. The metal layer 3 is not necessarily mademainly of copper. The layer may be made of any one of nickel, copper,silver, tin, gold, palladium, aluminum, chromium, titanium, and zinc, oran alloy of at least two of them.

The metal layer 3 may not be a metal foil formed by rolling. The layermay be metal foil formed by electrolysis (e.g., specialelectro-deposited copper foil) or may be formed by vacuum depositionwhich is an additive process, sputtering, chemical vacuum deposition,metal organic chemical vacuum deposition, or plating.

The plating may be electroplating (plating by electrolysis reactionutilizing electricity through an external electrode or the like) orelectroless plating (plating by chemical reaction not utilizingelectricity through an external electrode or the like, e.g.,displacement plating or chemical plating). In consideration of theproduction efficiency, preferably the electroplating is performed afterthe electroless plating as preparation. The electroless platingtypically involves troublesome pretreatments such as plating of theplated surface and catalytic reaction. In this regard, to simplify thepretreatments, the layer may be coated with conductive polymer. Apreferred but non-limiting example of the conductive polymer is catalystspecies such as palladium. The lower limit of the thickness of the metallayer 3 is further preferably 1 μm, even further preferably 2 μm. Toimprove the sliding characteristic, the upper limit of the thickness ofthe metal layer 3 is further preferably 6 μm, even further preferably 3μm.

The anisotropic conductive adhesive layer 4 is an anisotropic conductiveadhesive layer which is anisotropic as it is electrically conductiveonly in the thickness directions. The transmission characteristics ofthis layer are good as compared to an isotropic conductive adhesivelayer which is isotropic as it is electrically conductive in threedimensions, i.e., in all of the thickness directions, the widthdirections, and the longitudinal directions. The anisotropic conductiveadhesive layer 4 is formed by adding a flame retardant and conductivefillers to an adhesive.

When the shield film 1 is employed in a FPC (flexible printed wiringboard), the lower limit of the thickness of the anisotropic conductiveadhesive layer 4 is preferably 2 μm, more preferably 3 μm. The upperlimit of the thickness of the anisotropic conductive adhesive layer 4 ispreferably 15 μm, more preferably 3 μm.

The adhesive in the anisotropic conductive adhesive layer 4 is, asadhesive resin, constituted by thermosetting resin such as polystyreneresin, vinyl acetate resin, polyester resin, polyethylene resin,polypropylene resin, polyamide resin, rubber, and acrylic resin, and/orthermosetting resin such as phenol resin, epoxy resin, urethane resin,melamine resin, and alkyd resin. The adhesive may be made sorely of theresin above or a mixture of the above. The adhesive may further includetackifier. Examples of the tackifier include fatty acid hydrocarbonresin, C5/C9 mixed resin, rosin, rosin derivative, terpene resin,aromatic hydrocarbon resin, and thermal reactive resin.

The conductive fillers added to the anisotropic conductive adhesivelayer 4 are made of a metal material at least in part. For example, theconductive fillers are copper powder, silver powder, nickel power,silver-coated copper powder (Ag-coated Cu powder), gold-coated copperpowder, silver-coated nickel powder (Ag-coated Ni powder), orgold-coated nicked powder. These types of metal powders are formed byatomization, a carbonyl process, or the like. Alternatively, particlesformed by coating metal powder with resin or particles formed by coatingresin with metal powder may be used as the conductive fillers. To theanisotropic conductive adhesive layer 4, a mixture of more than one typeof conductive fillers 1 may be added. The conductive fillers arepreferably Ag-coated Cu powder or Ag-coated Ni powder. This is becauseconductive particles having stable conductivity are obtained at lowcost.

The amount of the conductive fillers to be added is 3 wt % to 39 wt % ofthe entire amount of the anisotropic conductive adhesive layer 4. Anaverage particle size of the conductive fillers preferably falls withinthe range of 2 μm to 20 μm. An optimal average particle size is chosenin consideration of the thickness of the anisotropic conductive adhesivelayer 4. The metal fillers may be spherical, needle-shaped, fibrous,flaky, or arborescent.

(Structure of Shielded Printed Wiring Board 10)

Now, referring to FIG. 2, a shielded printed wiring board 10 in whichthe above-described shield film 1 is joined with an FPC (flexibleprinted wiring board) will be described. Note that, although theembodiment deals with a case where the shield film is joined with theFPC, the disclosure is not limited to this arrangement. The shield filmmay be employed in a COF (Chip-on-Flex), an RF (Rigid Flexible PrintedBoard), a multilayer flexible board, and a rigid board.

As shown in FIG. 2, the shielded printed wiring board 10 is formed bydepositing the above-described shield film 1 on a base film (FPC) 8. Thebase film 8 is formed by deposing a basic film 5, a printed circuit 6,and an insulating film 7 one by one.

As shown in FIG. 2(a), the surface of the printed circuit 6 isconstituted by a signal circuit 6 a and a ground circuit 6 b. Theprinted circuit 6 is entirely covered with the insulating film 7 exceptat least at a part of the ground circuit 6 b (non-insulating part 6 c).The insulating film 7 includes an insulation removal part 7 a filledwith apart of the anisotropic conductive adhesive layer 4 of the shieldfilm 1. This allows the ground circuit 6 b to be electrically connectedto the metal layer 3.

The wiring patterns of the signal circuit 6 a and the ground circuit 6 bare formed by etching a conductive material. The ground circuit 6 bindicates a pattern retaining a ground potential. In other words, aground circuit 6 b which is a wiring pattern for grounding is formed inthe basic film 5. Alternatively, as shown in FIG. 2(b), the printedcircuit 6 may not include the ground circuit 6 b. In such a case, theprinted circuit 6 is entirely covered with the insulating film 7.

In the present embodiment, a signal with a frequency of 10 MHz to 10 GHzis supplied to the signal circuit 6 a. To put it differently, the shieldfilm 1 is preferably but not necessarily used as a shield film for asignal transmission system by which a signal with a frequency of 10 MHzto 10 GHz is transmitted.

The lower limit of the frequency of the signal transmission system forwhich the shield film 1 is used is preferably 10 MHz, more preferably100 MHz. The upper limit of the frequency of the signal transmissionsystem for which the shield film 1 is used is preferably 10 GHz, morepreferably 5 GHz.

The basic film 5 may be joined with the printed circuit 6 using anadhesive, or they may be joined with each other without using anadhesive, e.g., in a similar manner as so-called adhesive-freecopper-clad laminated sheets. In addition to the above, the insulatingfilm 7 may be a flexible insulating film joined using an adhesive, ormay be formed by a series of processes such as application ofphotosensitive insulating resin, drying, exposure, development, andthermal treatment. When the insulating film 7 is joined using anadhesive, an insulation removal part 7 a is formed also at a part of theadhesive corresponding to the ground circuit 6 b. Furthermore, the basefilm 8 is suitably selected from a one-sided FPC in which a printedcircuit is provided only on one side of a basic film, a double-sided FPCin which printed circuits are provided on the both sides of a basicfilm, a multilayer FPC in which these types of FPCs are laminated, aFlexboard (registered trademark) including a multilayer mounting portionand a cable portion, a flexible rigid board in which members forming amultilayer portion are hard, and a TAB tape for tape carrier package.

Both of the basic film 5 and the insulating film 7 are made ofengineering plastics. Examples of the engineering plastics includeresins such as polyethylene terephthalate, polypropylene, cross-linkedpolyethylene, polyester, polybenzimidazole, polyimide, polyimidoamide,polyetherimide, and polyphenylene sulfide (PPS). A low-cost polyesterfilm is preferred when the required degree of heat resistance is nothigh. A polyphenylene sulfide film is preferred when flame retardance isrequired. A polyimide film is preferred when good heat resistance isrequired.

The lower limit of the thickness of the basic film 5 is preferably 10μm, more preferably 20 μm. The upper limit of the thickness of the basicfilm 5 is preferably 60 μm, more preferably 40 μm.

The lower limit of the thickness of the insulating film 7 is preferably10 μm, more preferably 20 μm. The upper limit of the insulating film 7is preferably 60 μm, more preferably 40 μm.

(Manufacturing Method of Shield Film 1)

A method of manufacturing the shield film 1 of the present embodimentwill be described.

To begin with, copper is rolled by inserting the copper into the spacebetween rotating rollers, until the thickness becomes a first size. Thelower limit of this thickness in the first size is preferably 3 μm, morepreferably 6 μm, further preferably 3 μm. The upper limit of thethickness in the first size is preferably 35 μm, more preferably 18 μm,and further preferably 12 μm.

The copper foil rolled to have the thickness in the first size is etchedto be as thick as the second size (0.5 μm to 12 μm), so that a metallayer 3 is formed. More specifically, the copper foil which is 6 μmthick is immersed in etchant made up of sulfuric acid and hydrogenperoxide to be 2 μm thick. Note that, the adhesion of the etched copperfoil surface is preferably improved by plasma treatment.

Furthermore, a surface of the metal layer 3 is coated with theanisotropic conductive adhesive layer 4. With the other surface of themetal layer 3, an insulating layer 2 which is a protective film isjoined. The step of forming the insulating layer may be omitted.

(Method of Manufacturing Shielded Printed Wiring Board 10)

To begin with, an insulation removal part 7 a is formed in theinsulating film 7 of the base film 8 by boring a hole in the film 7 bylaser or the like. AS a result, a part of the ground circuit 6 b isexposed to the outside at the insulation removal part 7 a.

Subsequently, a shield film 1 is joined with the insulating film 7 ofthe base film 8. This joining is carried out in such a way that, whilethe shield film 1 is being heated by a heater, the printed wiring board10 and the shield film 1 are pressed against each other in verticaldirections by a pressing machine. As the anisotropic conductive adhesivelayer 4 of the shield film 1 is softened by the heat of the heater, theshield film 1 is joined with the insulating film 7 on account of thepressure of the pressing machine. At the same time, a part of thesoftened anisotropic conductive adhesive layer 4 fills the insulationremoval part 7 a. As a result, the part of the ground circuit 6 bexposed at the insulation removal part 7 a is joined with theanisotropic conductive adhesive layer 4. The ground circuit 6 b and themetal layer 3 therefore become electrically connected with each otherthrough the anisotropic conductive adhesive layer 4.

The embodiment of the present invention has been described above. It isnoted that the present invention is not necessarily limited to theembodiment above.

For example, the shield film 1 may not be attached to one side of theshielded printed wiring board 10 of the present embodiment. For example,shield films may be attached to both sides.

In the shielded printed wiring board 10 of the present embodiment, whenthe metal material of which the metal layer 3 is mainly made is copper,the surface of the metal layer 3 may be oxidized and the surfaceresistance may be increased, due to reasons in manufacturing conditionsor manufacturing steps. When the surface resistance is increased in thisway, the connection resistance between the anisotropic conductiveadhesive layer 4 and the ground circuit 6 b is also increased, with theresult that the shielding effect of the shield film 1 may bedeteriorated.

In consideration of this, as shown in FIG. 11, a protective metal layer3 a having low surface resistance and low connection resistance may beprovided between the metal layer 3 and the anisotropic conductiveadhesive layer 4. This protective metal layer 3 a is preferably made ofsilver (Ag) or gold (Au). The protective metal layer 3 a may be formedby vacuum deposition which is an additive process, sputtering, chemicalvacuum deposition, metal organic chemical vacuum deposition, or plating,in the same manner as the metal layer 3.

As such, on account of the protective metal layer 3 a provided betweenthe metal layer 3 and the anisotropic conductive adhesive layer 4, theoxidization of the metal layer 3 is restrained and the increase in thesurface resistance of the metal layer 3 is restrained, with the resultthat the shielding effect is stably exerted.

The above embodiment thus described solely serves as a specific exampleof the present invention, and the present invention is not limited tosuch an example. Specific structures of various means and the like maybe suitably designed or modified. Further, the effects of the presentinvention described in the above embodiment are not more than examplesof most preferable effects achievable by the present invention. Theeffects of the present invention are not limited to those described inthe embodiments described above.

EXAMPLES

Now, the present invention will be explained in a specific manner, withreference to Examples 1 to 5 and Comparative Examples 1 and 2 of theshield film of the present embodiment. In Examples 1 to 5 andComparative Examples 1 and 2, a shield film (measurement sample) 101shown in Table 1 was used. Table 1 shows a manufacturing methods andmaterials of the metal layer and whether the adhesive layer is ananisotropic conductive adhesive or an isotropic conductive adhesive.

TABLE 1 COMPARATIVE EXAMPLE EXAMPLE COMPARATIVE COMPARATIVE TYPE ITEMEXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 EXAMPLE 1 EXAMPLE 2ARRANGE- INSULATING 5 5 5 5 5 5 5 MENT LAYER(μm) TYPE AND ROLLED ROLLEDROLLED ROLLED ROLLED Ag ROLLED THICKNESS COPPER COPPER COPPER COPPERCOPPER DEPOSITION COPPER OF METAL FOIL FOIL FOIL FOIL FOIL 0.1 FOILLAYER(μm) 0.5 1 2 3 6 6 TYPE AND ANISO- ANISO- ANISO- ANISO- ANISO-ANISO- ISO- THICKNESS TROPIC TROPIC TROPIC TROPIC TROPIC TROPIC TROPICOF 9 9 9 9 9 9 9 CONDUCTIVE ADHESIVE LAYER(μm)

<Electric Field Waves and Magnetic Field Wave Shielding Characteristics>

To begin with, the electric field wave and magnetic field wave shieldingcharacteristics of the shield film were evaluated by the KEC methodusing an electromagnetic wave shielding effect measuring apparatus 11(electric field wave shielding effect evaluator 11 a and magnetic fieldwave shielding effect evaluator 11 b) developed by KEC ElectronicIndustry Development Center. FIG. 3 shows the structure of a system usedin the KEC method. The system used in the KEC method is constituted bythe electromagnetic wave shielding effect measuring apparatus 11, aspectrum analyzer 21, an attenuator 22 for attenuation by 10 dB, anattenuator 23 for attenuation by 3 dB, and a pre-amplifier 24.

As the spectrum analyzer 21, U3741 made by ADVANTEST CORPORATION wasused. Furthermore, HP8447F made by Agilent Technologies was used.

As shown in FIG. 3, a jig for measuring the electric field waveshielding characteristics is different from a jig for measuring themagnetic field wave shielding characteristics (measuring jigs 13 and15). FIG. 3(a) shows the electric field wave shielding effect evaluator11 a whereas FIG. 3(b) shows the magnetic field wave shielding effectevaluator 11 b. In the electric field wave shielding effect evaluator 11a, two measuring jigs 13 are provided to oppose each other. Betweenthese measuring jigs 13, a shield film (measurement sample) 101 which isthe measurement target shown in Table 1 is provided. Each measuring jig13 is sized in accordance with the TEM cell (Transverse ElectroMagneticCell), and is symmetrical about a plane which is orthogonal to thetransmission axis thereof. However, to prevent the occurrence ofshort-circuit as a result of the insertion of the measurement sample101, a planer central conductor 14 is spaced from each measuring jig 13.

In the electric field wave shielding effect evaluator 11 a, twomeasuring jigs 15 are provided to oppose each other. Between thesemeasuring jigs 15, the shield film 101 which is the measurement targetis provided. To generate an electromagnetic field with a large magneticfield wave component, the wave shielding effect evaluator 11 b isstructured such that a shield-type circular loop antenna 16 is used asthe measuring jig 15, and a quarter of the loop antenna juts to theoutside in combination with a metal plate with an angle of 90 degrees.

The shield films 101 in Example 3, Example 5, and Comparative Example 1shown in Table 1 for the measurement were sized 15 centimeters square.The measurement was done in the frequency range of 1 MHz to 1 GHz.Furthermore, the measurement was done at a temperature of 25 degreescentigrade and in the relative moisture of 30 to 50 percent.

In the KEC method, to begin with, a signal output from the spectrumanalyzer 21 is input to the measuring jig 13 or the measuring jig 15 onthe sender side, via the attenuator 22. After received by the measuringjig 13 or the measuring jig 15 on the receiver side and passing throughthe attenuator 23, the signal is amplified by the pre-amplifier 24, andthe signal level is measured by the spectrum analyzer 21. The spectrumanalyzer 21 outputs an attenuation amount when the shield film isprovided in the electromagnetic wave shielding effect measuringapparatus 11, on the basis of the state in which the shield film is notprovided in the electromagnetic wave shielding effect measuringapparatus 11.

Measurement results of the electric field wave shielding capabilityaccording to the KEC method are shown in FIG. 4(a), and measurementresults of the magnetic field wave shielding capability according to theKEC method are shown in FIG. 4(b). According to the results, theattenuation amounts are large in Examples 3 and 5 as compared toComparative Example 1, indicating that they are effective in terms ofthe shielding characteristics.

<Frequency Characteristics>

The frequency characteristics of the shield film were evaluated using anetwork analyzer 31 shown in FIG. 5. As the network analyzer 31, ZVL6made by Rohde & Schwarz was used. The network analyzer 31 includes aninput terminal and an output terminal, and connection boards 32 areconnected to the respective terminals. Between the pair of connectionboards 32, a shield flexible printed wiring board 110 which is themeasurement target is connected so as to linearly float in the air. Themeasurement was done in this state.

A measurement target (shield flexible printed wiring board 110 a) wasmanufactured in such a way that a shield flexible printed wiring board110 in which the printed circuit included no ground circuit(hereinafter, one-sided shielding) as shown in FIG. 2(b) wasmanufactured from each of the shield films 101 of Example 5, ComparativeExample 1, and Comparative Example 2 shown in Table 1. Anothermeasurement target (shield flexible printed wiring board 110 b) wasmanufactured by attaching the shield film 101 onto the basic film sideof the shield flexible printed wiring board 110 a (hereinafter,double-sided shielding). In these shield flexible printed wiring boards110, an insulating film which was 37.5 μm thick and was formed byjoining a polyimide film 12.5 μm thick with an adhesive layer 25 μmthick was used. The circuit pattern was formed by conducting 6 μm-copperplating on copper foil 12 μm thick. As described above, the circuitpattern does not include a ground circuit. As the basic film, apolyimide film 25 μm thick was used. The shield flexible printed wiringboard 110 was 200 mm in length. The measurement was done in thefrequency range of 100 kHz to 6 GHz. Furthermore, the measurement wasdone at a temperature of 25 degrees centigrade and in a relativemoisture of 30 to 50 percent.

The network analyzer 31 measures, for each frequency, to what extent anoutput signal is attenuated as compared to an input signal. Measurementresults of one-sided shielding by the network analyzer 31 are shown inFIG. 6(a), whereas measurement results of double-sided shielding by thenetwork analyzer 31 are shown in FIG. 6(b). According to the results,both in the one-sided shielding and in the double-sided shielding, theattenuation amount in Example 5 was small as compared to ComparativeExamples 1 and 2, indicating that it had good transmissioncharacteristics.

In regard to FIG. 6(a) and FIG. 6(b), attenuation amounts at typicalfrequencies in Comparative Examples 1 and 2 and Example 5 are shown inTable 2.

TABLE 2 SHIELDING FREQUENCY (MHz) SHIELD FILM METHOD 10 30 100 1000 6000COMPARATIVE EXAMPLE 1 ONE-SIDED −0.32 −0.68 −1.81 −11.95 −23.12COMPARATIVE EXAMPLE 2 SHIELDING −0.31 −0.43 −0.84 −8.23 −23.81 EXAMPLE 5−0.31 −0.42 −0.75 −6.17 −14.93 COMPARATIVE EXAMPLE 1 DOUBLE-SIDED −0.30−0.72 −2.75 −17.96 −28.78 COMPARATIVE EXAMPLE 2 SHIELDING −0.35 −0.57−2.01 −11.13 −27.77 EXAMPLE 5 −0.35 −0.52 −1.55 −9.12 −16.33 (UNIT: dB)

According to the measurement results in Table 2, around 30 MHz exceeding10 MHz, a difference in the attenuation amounts is observed betweencases where the metal layer is copper foil (Example 5 and ComparativeExample 2) and the cases where the metal layer is not a copper foil(Comparative Example 1), and this difference becomes conspicuous as thefrequency increases. Furthermore, at 6 GHz (6000 MHz), the attenuationamount in Comparative Example 1 is on a similar level as the attenuationamount in Comparative Example 2 employing an isotropic conductiveadhesive, but the attenuation amount is small in Example 5 employing ananisotropic conductive adhesive. As such, when the shield film of thepresent invention is used in a signal transmission system fortransmitting a signal with a frequency of 10 MHz to 10 GHz, electricfield waves, magnetic field waves and electromagnetic waves progressingfrom one side to the other side of the shield film are suitablyshielded.

<Output Wave Characteristics>

The output wave characteristics of the shield film were evaluated usinga system configuration shown in FIG. 7. The system was constituted by adata generator 41, an oscilloscope 42, a sampling module 43 attached tothe oscilloscope 42, and a pair of connection boards 32.

As the data generator 41, 81133A made by Agilent Technologies was used.As the oscilloscope 42, DSC8200 made by Tektronix, Inc. was used. As thesampling module 43, 80E03 made by Tektronix, Inc. was used.

As shown in FIG. 7, each connection board 32 includes an input terminaland an output terminal, and between the pair of connection boards 32, ashield flexible printed wiring board 110 which is the measurement targetis connected so as to linearly float in the air, and the data generator41 is connected with the sampling module 43. The eye pattern wasmeasured in this state.

Furthermore, the measurement was done using a device similar to theshielded printed wiring board 110 used for measuring the frequencycharacteristics. The input amplitude was set at 150 mV/side (300mVdiff). The data pattern was PRBS23. The measurement was done at atemperature of 25 degrees centigrade and in a relative moisture of 30 to50 percent.

Measurement results by the oscilloscope 42 at the bit rate of 1.0 Gbpsare shown in FIG. 8(a), and measurement results by the oscilloscope 42at the bit rate of 3.0 Gbps are shown in FIG. 8(b). According to theresults, at any bit rate and both in one-sided shielding and indouble-sided shielding, jitter was frequency observed in the eye patternof Comparative Examples 1 and 2 as compared to Example 5. This indicatesthat Example 5 is suitable for high-speed processing.

<Shape Retaining Property>

The shape retaining property of the shield film was evaluated. In thisconnection, a sample 51 was formed by attaching the shield film 101shown in Table 1 to each of the both surfaces of a polyimide film 50 μmthick. The sample 51 was sized 10 mm×100 mm.

As shown in FIG. 9, such a sample 51 was folded at a folding part 51 aat around the longitudinal center (around 50 mm) so that a fold isslightly formed, and an upper part 51 b above the folding part 51 aopposed to a lower part 51 c below the folding part 51 a.

The entirety of the sample 51 was placed on a PP (polypropylene)substrate 54, and SUS plates (not illustrated) each 0.3 mm thick wereprovided on the both sides of the sample 51 as spacers, to be inparallel to the length of the sample 51. In this state, silicon rubber53 was lowered from above and the entirety of the sample 51 was pressedtogether with the SUS plates. Because of the presence of the SUS plateseach 0.3 mm thick, the bend radius of the sample 51 was 0.15 mm at thefolding part 51 a. Both when the pressing force in the pressing was 0.1MPa and when the pressing force was 0.3 MPa, the pressing time was setat one second, three seconds, or five seconds, and the angle (returnangle) formed by the upper part 51 b and the lower part 51 c in thesample 51 after the pressing was measured.

In Table 3 below, the return angles measured in Examples 1 to 5 andComparative Examples 1 and 2 are shown. In regard to the evaluation, incase of double-sided shielding, an evaluation result was “Good” when theangle was 90 degrees or smaller, whereas an evaluation result was “Bad”when the angle was 120 degrees or larger. According to Table 3, theshape retaining property was good in rolled copper foil. This indicatesthat rolled copper foil is suitable in terms of the shape retainingproperty.

<Sliding Characteristic>

Under the IPC standard, as shown in FIG. 10, a shield flexible printedwiring board 111 (formed by using a sample of one of Examples 1 to 5 andComparative Examples 1 and 2) was bended to form a U-shape with thecurvature at 0.65 mm and was provided between a fixing plate 121 and asliding plate 122 that were distanced from each other by 1.30 mm, andunder a test atmosphere of 25 degrees centigrade in temperature and 30to 50 percent in relative moisture, the endurance of the metal layer ofthe shield film in the shield flexible printed wiring board (i.e., howmany times the sliding must be done to deteriorate the layer) wasmeasured when the sliding plate 122 was vertically slid with the strokelength of 50 mm (sliding area of 25 mm) and at the sliding speed of 60cpm. Each shield film was 140 mm long. Measurement results of Examples 1to 5 and Comparative Examples 1 and 2 are shown in Table 3 below,together with the shape retaining properties.

Also in Table 3, measurement results regarding the above-describedfrequency characteristics and shielding characteristics of Examples 1 to5 and Comparative Examples 1 and 2 are shown. In regard to the frequencycharacteristics, frequencies when the attenuation is at −3 dB and at −10dB are shown. In regard to the shielding characteristics, an attenuationamount with respect to an electric field wave of 1 GHz is shown.

TABLE 3 COMPARATIVE EXAMPLE COMPAR- COMPAR- EXAMPLE ATIVE ATIVE TYPEITEM EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 EXAMPLE 1 EXAMPLE2 EVALU- FREQUENCY 500 530 550 550 560 220 430 ATIONCHARACTERISTICS(MHz) 3500 3600 3660 3660 3660 760 1890 [@~3 dB/@~10 dB]SHIELDING 74 80 86 90 96 60 96 CHARACTERISTICS(dB) [1 GHz] SLIDING500,000 400,000 300,000 50,000 200 400,000 200 CHARACTERISTIC(times)R0.65 SHAPE RETAINING GOOD GOOD GOOD GOOD GOOD BAD GOOD PROPERTY

According to the results shown in Table 3, in regard to high-speedtransmission, for the same attenuation value, the frequency was high inthe case of the anisotropic conductive adhesive. This indicates that theattenuation did not occur even at a high frequency, and hence the use ofthe anisotropic conductive adhesive was clearly suitable for thehigh-speed processing.

In regard to the shielding characteristics, the shieldingcharacteristics were good when the thickness of the metal layer was 0.5μm or more.

It is therefore concluded that a shield film which is suitable forhigh-speed processing while maintaining shielding characteristics isrealized when the metal layer is at least 0.5 μm thick and theanisotropic conductive adhesive is used therein.

The sliding characteristic was significantly deteriorated when thethickness of the metal layer was 5 μm or more. Therefore, whenimportance is given to the sliding characteristic, it is clearlypreferable that the thickness of the metal layer is 5 μm or less.

In regard to the shape retaining property, good shape retaining propertywas obtained when the metal layer was a copper foil and formed byrolling. Therefore, when importance is given to the shape retainingproperty, rolled copper foil is clearly preferred.

<Connection Resistance>

The connection resistance between the shield film and the shieldedprinted wiring board after the manufacturing step of manufacturing theshielded printed wiring board 10 was measured (measurement of connectionresistance after reflow). More specifically, as shown in Table 4, inExample 6, the copper foil was not treated with rust prevention, and theconnection resistance (Ω) between the shield film 1 and the groundcircuit 6 b was measured using the shielded printed wiring board 10 inwhich a protective metal layer 3 a formed by vacuum-depositing silver tobe 0.05 μm thick was provided between the metal layer 3 and theanisotropic conductive adhesive layer 4 of the shield film 1. In Example7, the copper foil was not treated with rust prevention, and theconnection resistance (Ω) between the shield film 1 and the groundcircuit 6 b was measured using the shielded printed wiring board 10 inwhich a protective metal layer 3 a formed by vacuum-depositing silver tobe 0.1 μm thick was provided between the metal layer 3 and theanisotropic conductive adhesive layer 4 of the shield film 1. In Example8, the copper foil was not treated with rust prevention, and theconnection resistance (Ω) between the shield film 1 and the groundcircuit 6 b was measured using the shielded printed wiring board 10 inwhich a protective metal layer 3 a formed by plating silver to be 0.05μm thick was provided between the metal layer 3 and the anisotropicconductive adhesive layer 4 of the shield film 1. In Example 9, thecopper foil was not treated with rust prevention, and the connectionresistance (Ω) between the shield film 1 and the ground circuit 6 b wasmeasured using the shielded printed wiring board 10 in which aprotective metal layer 3 a formed by plating silver to be 0.1 μm thickwas provided between the metal layer 3 and the anisotropic conductiveadhesive layer 4 of the shield film 1. In Example 5, the copper foil wastreated with rust prevention, and the connection resistance (Ω) betweenthe shield film 1 and the ground circuit 6 b was measured in a shieldedprinted wiring board not including a protective metal layer 3 a. Each ofthe shield films 1 in Examples 5 to 9 was arranged such that theinsulating layer 2 was 5 μm thick, the metal layer 3 (rolled copperfoil) was 6 μm thick, and the anisotropic conductive adhesive layer 4was 3 μm thick. Each of the shield films 1 of Examples 6 to 9 wasarranged such that the protective metal layer 3 a was further providedbetween the metal layer 3 and the anisotropic conductive adhesive layer4.

TABLE 4 EXAMPLE5 EXAMPLE6 EXAMPLE7 EXAMPLE8 EXAMPLE9 TREATMENT OF COPPERFOIL RUST NONE NONE NONE NONE PREVENTION PROTECTIVE TYPE NONE SILVERSILVER SILVER SILVER METAL LAYER THICKNESS(μm) NONE 0.05 0.1 0.05 0.1METHOD NONE VAPOR VAPOR PLATING PLATING DEPOSITION DEPOSITION CONNECTIONRESISTANCE(Ω) 1.45 0.52 0.36 0.03 0.02

According to the measurement results in Table 4, the connectionresistance in Example 5 was endurable to practical use as it was 2Ω orlower, but the connection resistance was further reduced when theprotective metal layer 3 a was provided as in Examples 6 to 9.Furthermore, comparing Example 6 with Example 8 and comparing Example 7with Example 9, the connection resistance was low when the protectivemetal layer 3 a was formed by plating as compared to cases where thelayer 3 a was formed by vacuum deposition. Furthermore, comparingExample 6 with Example 8 and comparing Example 7 with Example 9, theconnection resistance was low when the protective metal layer 3 a wasthick.

REFERENCE SIGNS LIST

-   1 shield film-   2 insulating layer-   3 metal layer-   4 anisotropic conductive adhesive layer-   5 basic film-   6 printed circuit-   6 a signal circuit-   6 b ground circuit-   6 c non-insulating part-   7 insulating film-   7 a insulation removal part-   8 base film-   10 shielded printed wiring board

The invention claimed is:
 1. A shield film suitable for use in shieldinga signal-transmitting circuit from signal-interfering electromagneticnoise, comprising: a metal layer, which is 2 μm to 12 μm thick; aninsulating layer, which is 1 μm to 10 μm thick and is disposed along andin contact with a first surface of the metal layer; and an anisotropicconductive adhesive layer containing conductive filler with an averageparticle size of 2 μm to 20 μm, which anisotropic conductive adhesivelayer is disposed along and in contact with a second, opposite surfaceof the metal layer; wherein the shield film exhibits between 74 dB and96 dB of attenuation of an electric field wave at 1 GHz, as measuredusing KEC method.
 2. The shield film according to claim 1, wherein themetal layer is metal foil.
 3. The shield film according to claim 2,wherein the metal foil has been formed by rolling.
 4. The shield filmaccording to claim 3, wherein the thickness of the metal foil has beenadjusted by etching.
 5. The shield film according to claim 4, whereinthe metal foil is mainly made of copper.
 6. The shield film according toclaim 1, wherein the metal layer has been formed by an additive process.7. The shield film according to claim 6, wherein as the additiveprocess, the metal layer has been formed by at least one ofelectroplating and electrodeless plating.
 8. The shield film accordingto claim 1, wherein the shield film is configured to function as anelectromagnetic waves shield film of a signal transmission systemtransmitting a signal with a frequency of 10 MHz to 10 GHz.
 9. Ashielded printed wiring board, comprising: a printed wiring boardincluding a base member in which a printed circuit is formed and aninsulating film provided on the base member so as to cover the printedcircuit; and a shield film according to claim 1 provided on the printedwiring board.
 10. The shielded printed wiring board according to claim9, wherein the printed circuit includes a ground wiring pattern.
 11. Theshield film according to claim 1, wherein the metal layer is mainly madeof copper and includes a protective metal layer that is made of silveror gold.
 12. The shield film according to claim 11, wherein theanisotropic conductive adhesive layer is disposed in contact with theprotective metal layer.
 13. A shield film suitable for use in shieldinga signal-transmitting circuit from signal-interfering electromagneticnoise, comprising: a metal layer, which is 2 μum to 12 μm thick; aninsulating layer, which is 1 μm to 10 μm thick and is disposed along andin contact with a first surface of the metal layer; and an anisotropicconductive adhesive layer containing conductive filler with an averageparticle size of 2 μm to 20 μm, which anisotropic conductive adhesivelayer is disposed along and in contact with a second, opposite surfaceof the metal layer; wherein the shield film exhibits between about 70 dBand about 85 dB of attenuation of a magnetic field wave at 1 GHz, asmeasured using KEC method.